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Polishing slurry and polishing method therefor

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Polishing slurry and polishing method therefor


The present invention provides a polishing technique which enables polishing of silicon carbide, which is difficult to be polished, with high efficiency and high surface accuracy. The present invention relates to a polishing slurry for polishing a substrate, which comprises abrasive particles containing manganese oxide as a main component and in which the content of the abrasive particles is less than 10% by weight based on the polishing slurry. The polishing slurry of the present invention has a pH of preferably 7 or more. It is particularly preferable to use manganese dioxide as abrasive particles. The polishing slurry of the present invention is suitable for a substrate of silicon carbide.
Related Terms: Manganese Manganese Dioxide Silicon Silicon Carbide

Inventors: Yasuhide Yamaguchi, Mikimasa Horiuchi
USPTO Applicaton #: #20130012102 - Class: 451 36 (USPTO) - 01/10/13 - Class 451 
Abrading > Abrading Process >Utilizing Fluent Abradant

Inventors:

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The Patent Description & Claims data below is from USPTO Patent Application 20130012102, Polishing slurry and polishing method therefor.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing slurry containing manganese oxide as a main component and a method for polishing therewith, and particularly to a polishing slurry suitable for polishing silicon carbide.

2. Description of the Related Art

Recently polishing has been often used as means for surface processing of constituent materials of various electronic and electric products. In such polishing, surfaces of objects to be polished, such as substrates, are polished with abrasive particles dispersed in an aqueous liquid, namely, a polishing slurry. It is known that the amount of polishing at the time of polishing depends on the concentration of abrasive particles.

A larger amount of abrasive particles in polishing increases the contact frequency between abrasive particles and the surface of the object to be polished, and so the abrasive particles cut away more substances from the surface of the object to be polished, thereby increasing polishing rates. Controlling the concentration of abrasive particles in polishing has been applied to polishing slurries with abrasive particles such as silicon oxide (SiO2) and aluminum oxide (Al2O3). It is common technical knowledge that the concentration of abrasive particles in such a polishing slurry, i.e., polishing slurry concentration, is set at 10% by weight to 20% by weight to carry out polishing. It has also been proposed that even in the case of polishing with, for example, manganese oxide as abrasive particles, the polishing slurry concentration is set at 10% by weight to 20% by weight (See Patent Document 1, Patent Document 2).

Recently, silicon carbide (SiC) has been attracting attention as a substrate material for power electronics semiconductors and white LEDs, and silicon carbide is known to be difficult to be machined due to its extremely high hardness. Given that, silicon carbide is polished with silicon oxide abrasive particles having excellent polishing properties. However, although the surface that has been polished has high surface accuracy, the polishing rate is low, and it is said that efficient polishing is difficult. Thus at present there is a strong demand for polishing technique capable of rapidly polishing even difficult-to-machine materials such as silicon carbide while achieving desired surface accuracy.

PRIOR ART DOCUMENT Patent Documents

[Patent Document 1] Japanese Patent Application Laid-Open No. 9-22888

[Patent Document 2] Japanese Patent Application Laid-Open No. 10-60415

SUMMARY

OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made under the above circumstances and provides a polishing technique capable of increasing polishing rates in polishing with a polishing slurry in which manganese oxide is used as abrasive particles. An object of the present invention is to provide a polishing technique capable of polishing, in particular, an object to be polished having high hardness and difficult to be machined, such as silicon carbide (SiC), at high polishing rates with good surface accuracy.

Means for Solving the Problems

The present inventors have conducted intensive studies on a polishing slurry prepared by dispersing manganese oxide in an aqueous liquid as abrasive particles, and have found that even at low concentration of abrasive particles, the polishing rate can be increased by chemical properties of the abrasive particles, and thus have accomplished the present invention.

The present invention relates to a polishing slurry for polishing a substrate, which comprises abrasive particles containing manganese oxide as a main component and in which the content of the abrasive particles is less than 10% by weight based on the polishing slurry. Although the content of the abrasive particles in the polishing slurry of the present invention is as low as less than 10% by weight, when manganese oxide is used as abrasive particles, the polishing rate is high and polishing offers a smooth polishing surface. The present invention is capable of forming a polishing surface having good surface accuracy at high polishing rates even at a polishing particle concentration lower than that of a conventional polishing slurry containing silicon oxide (SiO2). The polishing slurry of the present invention enables polishing of an object to be polished having high hardness and difficult to be machined, such as silicon carbide (SiC), at high polishing rates with good surface accuracy. In the present invention, abrasive particles containing manganese oxide as a main component mean that abrasive particles contain 90% by weight or more of manganese oxide.

When the content of the abrasive particles in the polishing slurry of the present invention exceeds 10% by weight, the polishing rate will increase, but the polishing surface will have low surface accuracy. The lower limit of the content is 0.1% by weight or more. This is because when the content is less than 0.1% by weight, the polishing rate is low and practical polishing is difficult. The content of the abrasive particles is more preferably 0.5% by weight to 5% by weight. The aqueous liquid in the polishing slurry of the present invention means water or a mixture of water and at least one organic solvent soluble in water mixed within the soluble range, containing at least 1% of water. Examples of organic solvents include alcohol and ketone.

Examples of alcohols that can be used in the present invention include methanol (methyl alcohol), ethanol (ethyl alcohol), 1-propanol (n-propyl alcohol), 2-propanol (iso-propyl alcohol, IPA), 2-methyl-1-propanol (iso-butyl alcohol), 2-methyl-2-propanol (tert-butyl alcohol), 1-butanol (n-butyl alcohol) and 2-butanol (sec-butyl alcohol). Examples of polyhydric alcohol include 1,2-ethanediol (ethylene glycol), 1,2-propanediol (propylene glycol), 1,3-propanediol (trimethylene glycol) and 1,2,3-propanetriol (glycerol)

Examples of ketones that can be used in the present invention include propanone (acetone) and 2-butanone (methyl ethyl ketone, MEK). In addition, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and 1,4-dioxane may also be used.

The polishing slurry of the present invention preferably has a pH of 7 or more. A pH of 7 or more offers a high polishing rate with maintaining good surface accuracy. More specifically, when the pH is 7 or more and the object to be polished is silicon carbide, polishing with a surface roughness Ra of the polishing surface of 0.2 nm or less at a polishing rate of 100 nm/hr or more is achieved. The upper limit of pH is 13. When the pH is more than 13, chemical properties of abrasive particles begin to change, that is, action of etching of silicon carbide due to manganese oxide begins, and the polishing surface is more likely to be roughened. The pH is preferably 7 to 12. When adjusting the pH, chemicals for that are not particularly limited. To suppress the negative effect on the object to be polished, however, potassium salts and ammonium salts are preferably used, and potassium salts are more preferably used.

In the polishing slurry of the present invention, manganese dioxide is preferably used as manganese oxide. Using manganese dioxide as abrasive particles offers a high polishing rate with maintaining good surface accuracy even when silicon carbide is the object to be polished. When manganese dioxide as abrasive particles is dispersed in water, the resultant has a pH of 5 to 6, and so for adjusting the pH to 7 or more, an alkaline chemical is preferably added thereto.

The particle size of manganese oxide that serves as abrasive particles is not particularly limited. To achieve good surface accuracy, 50% diameter D50 in volume-based cumulative fractions of particle diameter distribution measured with a laser diffraction/scattering method is preferably 1 μm or less, more preferably 0.5 μm or less.

In the present invention, objects to be polished are not particularly limited. Materials having high hardness and difficult to be machined are suitable as the object to be polished. Examples thereof include aluminum oxide (Al2O3), gallium nitride (GaN) and silicon carbide (SiC). It is particularly suitable that the object to be polished is silicon carbide (SiC).

Effect of the Invention

As described above, the polishing slurry of the present invention enables polishing of an object to be polished having high hardness and difficult to be machined, such as silicon carbide (SiC), at high polishing rates with good surface accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relation between slurry concentrations and polishing rates relative to the amount of abrasive particles.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described with reference to Examples and Comparative Examples.

Examples 1 to 4

In Examples 1 to 4, MnO2 having an average particle size D50 of 0.5 μm was used as abrasive particles, and the particles were dispersed in water as an aqueous liquid to prepare polishing slurries having a respective slurry concentration shown in Table 1. The polishing slurries of Examples 1 to 4 had a pH of 7.8. The average particle size D50 of MnO2 was measured with Laser Diffraction/Scattering Method Particle Size Distribution Analyzer (LA920 made by Horiba, Ltd.).

Silicon carbide single crystal substrates were polished with the respective polishing slurries to examine their polishing properties. The silicon carbide single crystal substrates which were the object to be polished were SiC single crystal (6H structure) of 2 inches in diameter and 330 μm in thickness. The polishing surface was on axis (the wafer surface cut perpendicularly to the crystal axis). Before polishing, the average surface roughness in a 10 μm×10 μm area of the surface to be polished of the substrate was measured with AFM (Atomic force microscopy: NanoScope IIIa made by Veeco Instruments Inc.). As a result, Ra was 2.46 nm.

Referring to the polishing condition, a silicon carbide single crystal substrate placed on a polishing pad (SUBA400 made by Nitta Haas Incorporated) was polished with the respective polishing slurries of Examples 1 to 4 at a polishing load of 250 g/cm2 for 3 hours. After the polishing, the polishing surface was washed with water to remove slurry attached and dried. The surface roughness of the dried polishing surface was measured at 5 random points by AFM. The results of the measurement of the average surface roughness (10 μm×10 μm area) are shown in Table 1. The weight of the silicon carbide single crystal substrate was measured before and after the polishing, and the weight difference was determined to be the amount of polishing, and with this, the polishing rate was calculated from the surface area and the specific gravity of the substrates. The polishing rates are shown in Table 1.

For comparison, polishing slurries having a slurry concentration of 10% by weight or more were prepared (Comparative Examples 1 to 3) and polishing slurries with conventional commercially available colloidal silica (made by Fujimi Incorporated, Compol 80 (silicon oxide (SiO2) abrasive)) were prepared (Comparative Examples 4 to 10). The colloidal silica had an average particle size D50 of 0.10 μm. In Comparative Examples 4 to 10, colloidal silica was dispersed in water as an aqueous liquid to prepare polishing slurries having a respective slurry concentration shown in Table 1.

Polishing properties were examined in the same conditions as in the above Examples 1 to 4. The polishing slurries of Comparative Examples 1 to 3 had a pH of 8.2 and the polishing slurries of Comparative Examples 4 to 10 had a pH of 8.7 to 9.1.

TABLE 1

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stats Patent Info
Application #
US 20130012102 A1
Publish Date
01/10/2013
Document #
13634939
File Date
11/22/2010
USPTO Class
451 36
Other USPTO Classes
51309
International Class
/
Drawings
2


Manganese
Manganese Dioxide
Silicon
Silicon Carbide


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